1. Field Of the Invention
This invention relates to an image recording apparatus and, more particularly, to an image recording apparatus that includes a multi-element head having a plurality of recording elements.
2. Description of the Prior Art
With the popularization of computers and communication apparatuses, the application of recording apparatuses for effecting digital image recording with an ink jet type or thermal transfer type recording head has rapidly been promoted. As recording heads for image forming apparatuses, multi-element heads having a plurality of image recording elements integrally combined are generally used for the purpose of increasing recording speed. For example, ink jet recording heads are generally constructed as multi-nozzle heads having a plurality of nozzles integrally combined. Also, thermal transfer type heads are usually constructed by combining a plurality of heaters.
However, it is difficult to uniformly manufacture a multi-element head having a plurality of image recording elements, and the characteristics of the image recording elements vary to some extent. For example, in an ink jet multi-element head, there are variations in the shape of the nozzles. In a thermal transfer multi-element head, there are variations in the shape and the resistance of the heaters. Non-uniformity of the characteristics of image recording elements appears as non-uniformity of the size or the density of dots recorded by the image recording elements, so that the recorded image is uneven in density.
To cope with this problem, various methods for obtaining a uniform image have been proposed which are based on changing signals supplied to the image recording elements so as to correct such unevenness. For example, in the case of a multi-element head having recording elements 2 arranged as shown in FIG. 5(a), density unevenness may occur as shown in FIG. 5(c) when the input signal level is constant as shown in FIG. 5(b) . In this case, the input signal level is changed for compensation as shown in FIG. 5(d); the input signal is supplied at a higher level to the recording elements corresponding to a low-density portion while the input signal is supplied at a lower level to the recording elements corresponding to a high-density portion. In the case of a recording system capable of changing the diameter or density of dots, the diameter of dots recorded by each recording element is changed in accordance with the input signal level. For example, the drive voltage or the width of pulses applied to each piezoelectric element in the case of piezoelectric ink jet printing or to each heater in the case of thermal transfer printing is changed in accordance with the input signal so that the dot diameters or the dot densities determined by the recording elements are generally equal, and so that the distribution of density of the recorded image is made uniform as shown in FIG. 5(e). In a case where it is impossible or difficult to change the dot diameter or dot density, the number of dots is changed according to the input signal in such a manner that a greater number of dots are formed by the image recording elements corresponding to a low-density portion while a smaller number of dots are formed by the recording elements corresponding to a high-density portion, thereby making the density distribution uniform, as shown in FIG. 5(e).
The following is an example of a method of determining the amount of this compensation.
A case of density unevenness correction for a multi-element head having 256 nozzles will be described below.
It is assumed here that the distribution of density unevenness in the case of recording using a uniform image signal S is as shown in FIG. 6. First, the mean density OD of an image formed by this head is obtained. Next, the density OD.sub.1 to OD.sub.256 of portions corresponding to the nozzles are measured. The deviation for each nozzle, .DELTA.OD.sub.n, is obtained using the formula: .DELTA.OD.sub.n =OD-OD.sub.n (n=1 to 256). If the relationship between the level of the image signal and the output density is as shown in FIG. 7, the image signal may be changed by .DELTA.S to correct the density by .DELTA.OD.sub.n. For this correction, the image signal may be changed by table conversion as shown in FIG. 8. In FIG. 8, the line A is a straight line having a slope or inclination of 1.0. With respect to the straight line A, the input signal is output without being changed. With respect to the line B, which is a straight line having a slope smaller than that of the line A, an output S-.DELTA.S is obtained from an input Accordingly, the image signal supplied to the nth nozzle may be changed by table conversion as indicated by the line B in FIG. 8 before driving the head, whereby the density of the portion printed by this nozzle is made equal to OD. If this processing is performed with respect to all the nozzles, the density unevenness is corrected and an uniform image can be obtained. That is, data for suitable conversion tables for compensation of image signals with respect to the nozzles is prepared to enable correction of such unevenness.
Although density unevenness can be suitably corrected by this method for at least the initial usage of the device, it is necessary to change the amount of compensation of the input signal for correcting the unevenness, if the degree of density unevenness is changed after time. In the case of an ink jet head, the density distribution is usually changed by deposits of ink or extraneous material to a nozzle portion in the vicinity of the ink outlet. In the case of a thermal transfer head, as well, there is a possibility of a change in the density distribution due to deterioration or change in the characteristics of each heater. In such cases, the amount of input correction initially set for density unevenness correction becomes insufficient, and the density unevenness becomes more conspicuous with time.
A method has been proposed of providing a density unevenness reader section in an image recording apparatus and revising density unevenness correction data by periodically reading the density unevenness distribution with the reader. In this method, the correction data is revised according to the change in the density unevenness distribution of the head to constantly maintain the desired uniformity of the recorded image free from density unevenness.
Correction data is prepared with respect to each recording apparatus actually used, and it is therefore necessary to set the period of time for preparing correction data to a very short time to limit the down time of the apparatus.
However, the effect of correction is unsatisfactory if reading of density unevenness and formation of correction data are performed only one time, and, in most cases, the uniformity of the recording image cannot be improved as desired unless these operations are repeated several times or ten and several times. The cause of this fact is a change in a variation in a gradation characteristic due to a change in the temperature of the head, which is caused as described below. In a case where the head has a gradation characteristic such as that indicated by the line C in FIG. 9, a degree .DELTA.OD of density unevenness can be corrected by changing the image signal S by .DELTA.S, as described above. However, the gradation characteristic of the recording head depends upon the temperature of the head. For example, in the case of ink jet recording, if the temperature of the head is increased, the viscosity of the ink is reduced, so that the amount of ink is increased while the ejection energy is constant, resulting in an increase in dot diameter. That is, a gradation characteristic such as that indicated by the line D in FIG. 9 is exhibited. When the head has this gradation characteristic, and when correction of .DELTA.S is effected based on detecting density unevenness .DELTA.OD, the amount of density unevenness correction is only .DELTA.OD', and a uniform image cannot be obtained by performing the correction operation only one time.
Thus, the necessary amount of correction is changed as the gradation characteristic of the head varies. The density unevenness cannot be sufficiently corrected by effecting reading and correction one or two times, and it is necessary to repeat the same operation many times.